In recent years, CO2 emissions have been strictly regulated due to growing environmental issues. In the automotive field, improvements in fuel efficiency by reduction in weight of automobile bodies are significant challenges. Therefore, weight reduction by applying high-strength steel sheets to automobile parts is in progress. In particular, high-strength steel sheets with a tensile strength (TS) of 1,180 MPa or more are applied to automobile parts.
High-strength steel sheets for use in automobile parts such as structural members and reinforcing members for automobiles are required to have excellent formability. In particular, a high-strength steel sheet for use in parts with a complicated shape is required to have both excellent elongation and stretch flangeability (also referred to as hole-expandability) rather than either one. Furthermore, automobile parts such as structural members and reinforcing members are required to have excellent impact energy absorption capability. Increasing the yield ratio of a steel sheet used is effective in enhancing the impact energy absorption capability thereof. Automobile parts manufactured using a steel sheet with high yield ratio can efficiently absorb impact energy with low deformation. Herein, the yield ratio (YR) is a value representing the ratio of the yield stress (YS) to the tensile strength (TS) and is given by the equation YR=YS/TS.
Dual-phase steels (DP steels) with a ferrite-martensite microstructure are conventionally known as high-strength steel sheets having high strength and formability. DP steel is multi-phase steel in which ferrite is a primary phase and martensite is distributed. DP steel has low yield ratio, high TS, and excellent elongation. However, DP steel has a disadvantage that stress is likely to concentrates at the interface between ferrite and martensite during deformation to cause cracks and therefore the stretch flangeability is low. As DP steel excellent in stretch flangeability, Japanese Unexamined Patent Application Publication No. 2011-052295 discloses a technique wherein a dual-phase microstructure is composed of tempered martensite and ferrite, the balance between elongation and stretch flangeability is ensured and a high strength of TS 1,180 MPa or more is achieved by controlling the hardness and area fraction of tempered martensite and the distribution of cementite grains in tempered martensite.
A TRIP steel sheet based on the transformation-induced plasticity of retained austenite is cited as a steel sheet having high strength and excellent ductility. TRIP steel sheets have microstructures containing retained austenite. In deforming a TRIP steel sheet at a temperature not lower than the martensite transformation start temperature, retained austenite is induced to transform into martensite by stress, whereby a large elongation is achieved. However, TRIP steel sheets have problem with poor stretch flangeability (stretch flangeability) because retained austenite is transformed into martensite during punching and therefore cracks are caused at the interfaces between ferrite and martensite. As a TRIP steel sheet with excellent stretch flangeability, Japanese Unexamined Patent Application Publication No. 2005-240178 discloses a low-yield ratio, high-strength cold-rolled steel sheet which has a microstructure containing at least 5% retained austenite, at least 60% bainitic ferrite, and 20% or less (including 0%) polygonal ferrite, which is excellent in elongation and stretch flangeability, and which has high strength, a TS of 980 MPa or more. Japanese Unexamined Patent Application Publication No. 2011-047034 discloses a high-strength steel sheet in which the area fraction of ferrite, bainite, and retained austenite is regulated; which has a microstructure with a martensite area fraction of 50% or more; in which the hardness distribution of martensite is controlled; and which has a TS of 980 MPa or more, excellent elongation, and excellent stretch flangeability.
However, steels such as DP steels based on martensite transformation generally have low yield ratio and reduced impact energy absorption capability because mobile dislocations are introduced into ferrite during martensite transformation. The steel sheets disclosed in JP '295 are insufficient in formability, particularly elongation. The steel sheets disclosed in JP '178 have a high strength of 980 MPa or more and, however, have no enhanced elongation or stretch flangeability in a high-strength range of 1,180 MPa or more. The steel sheets disclosed in JP '034 are insufficient in elongation and stretch flangeability.
As described above, in steel sheets with a high strength of 1,180 MPa or more, it is difficult that high yield ratio is maintained and excellent elongation and stretch flangeability are ensured such that excellent impact energy absorption capability is achieved. Therefore, the development of a steel sheet having these properties has been desired.
It could therefore be helpful to provide a high-strength cold-rolled steel sheet having excellent elongation, excellent stretch flangeability, and high yield ratio and a method of manufacturing the same.